Currently the field your reference is only visible on the Sales Quote. It is transfered to the Order and Invoice (posted) but not visibile. On reports Your reference is default available in the layout data set.

External document No. is not available in the default data set. Therefor you cannot print the customers external document No. in the Posted invoice.

This means we have a field which is not visible but printable and we have a field which is visible but not printable.

Please add either your reference to the page or external document no to the default datasets.